The present invention relates to a new class of water-developable photopolymerizable compositions, and photosensitive elements and photopolymer printing plates that utilize such compositions. More specifically, the novel photopolymerizable compositions of the present invention can be used without any pre-exposure conditioning of the type that has become customary with various prior art compositions. The present compositions thus provide photosensitive elements and photopolymer printing plates that not only have vastly improved printing characteristics, but can be directly subjected to an imagewise exposure and then developed with water without any costly or time consuming pre-exposure conditioning steps.
Many photopolymerizable compositions and photosensitive elements that are useful in making printing plates of one kind or another are known in the art. One, if not the most, commercially successful of all such prior art compositions is described in U.S. Pat. No. 3,801,328 to Takimoto et al. Such compositions, although they have met with enormous commercial success, nevertheless require some form of pre-exposure conditioning before imagewise exposure in order to produce commercially satisfactory relief images on printing plates that employ such compositions.
Two techniques for pre-exposure conditioning have evolved in the art. The first, commonly known as CO.sub.2 conditioning, requires the maintenance of the photosensitive element or plate in a CO.sub.2 atmosphere until just prior to the imagewise exposure of the element or plate. The second known technique, which has supplanted the CO.sub.2 conditioning technique and is commonly known as "bump exposure", requires that the photosensitive element or plate be subjected to a brief (typically 2 to 8 second) light exposure immediately prior to the longer duration imagewise exposure, in the presence of a negative.
It has been recognized, of course, that both pre-exposure conditioning techniques require special, costly and time consuming additional handling of the photosensitive element or plate that, from a plate processing standpoint should desirably be eliminated. Moreover, both techniques make the design and operation of automatic plate exposure and processing equipment unduly complicated and expensive. And perhaps even more importantly, both techniques, because of the necessity that imagewise exposure be carried out within a designated time period after pre-exposure conditioning occurs, introduce time controls in plate processing that are oftentimes difficult, if not impossible, to carefully maintain in the large quantity, commercial newspaper applications for such plates.
It has been determined by the applicants herein, for example, that a printing plate that has been "bump" exposed should desirably be subjected to an imagewise exposure within approximately 30 seconds after the bump exposure is completed. Variations in this period of plate dormancy, between the completion of the bump exposure step and the initiation of the imagewise exposure steps can, and often does, produce developed plates having vastly different printing quality characteristics. Thus, two plates utilizing the same photopolymer composition and same negative can have different printing characteristics simply because the period of dormancy between bump and imagewise exposure varies.
Moreover, photosensitive elements and plates, which are subjected to bump exposure, must typically be exposed under high intensity point sources of actinic light, such as carbon are lamps or high pressure mercury vapor lamps. Such lamps not only have extremely high current demands and short lives, but in normal operation produce unwanted heat, which can cause or contribute to the degradation of the photopolymerizable compositions or cause negatives to stick to the plate photopolymer surface.
As a result of the many inconveniences, the added expenses and variations in product quality inherent in the use of plates requiring some form of pre-conditioning, either through CO.sub.2 conditioning or bump exposure, a need has developed for water-developable photopolymerizable compositions (and resultant elements and plates) that entirely eliminate the necessity of pre-exposure conditioning, yet at the same time provide all of the advantageous properties of the commercially desirable compositions of the type described in the Takimoto U.S. Pat. No. 3,801,328 patent. One such effort to solve the pre-exposure conditioning problem is described in U.S. Pat. No. 4,042,386 to Okai and Kimoto, wherein the selection and use of specific components, in certain ratios and amounts, and intensified exposure of the resulting photosensitive element or plate, are disclosed as means of eliminating the need for pre-exposure conditioning. This approach, however, suffers from the disadvantage that only carefully selected components and ratios amounts of such components can be used, and from the additional disadvantage that only costly high intensity light sources (which have shortened lamp lives and generate excessive amounts of undesired heat) can be used. Moreover, it has been determined that the photopolymerizable compositions disclosed in the Okai and Kimoto patent work best with lower reflectivity substrates (e.g., a high degree of antihalation) in order to provide developed photopolymerizable elements having the desired dot depth characteristics in highlight, middle tone and shadow image areas.